Opposition leader Belden Namah on Sunday announced lawyers acting on his behalf had filed a challenge to the detention centre on Friday.

But Mr O'Neill says he is confident the controversial detention centre complies with PNG law.

"We will defend it vigorously," he told reporters in Port Moresby on Monday.

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"On advice from the attorney-general's office - our principle and the only advice the (cabinet) or government can take is from his office - the office says we are in compliance of our laws and our obligations to international conventions we have signed."

The injunction seeks to have the current detainees released and to prevent the government from receiving or detaining any more asylum seekers from Australia.

Mr Namah said in a statement that he regretted taking the action against the PNG government, but he believed the processing centre was unconstitutional.

"The ministers of the O'Neill-Dion government have now received a summons to appear and defend their conduct in the national court," he said in the statement.

Mr Namah said the detainees on Manus were being held illegally in PNG.

"We will take this matter as far as necessary to ensure that the values of our nation's constitution are upheld," he said.

"This legal challenge also attempts to remedy the many abuses of PNG law and of ministerial powers which have given rise to the situation on Manus."

He said the opposition challenged the right of the government to force people seeking refugee status in Australia to enter PNG, where they were being held "illegally and indefinitely under inhumane conditions".

Manus MP Ronny Knight - a member of Mr O'Neill's government - accused Mr Namah of trying to score political points.

"If the shoe was on the other foot and he was prime minister, he would be pushing for Manus," Mr Knight told AAP.

The motivation for the court challenge has also drawn comment in Australia.

"I think when you've got the opposition leader in Papua New Guinea bringing a proceeding in the Supreme Court ... it does smack of politics," Mr Dreyfus told Sky News on Monday.

Both Mr O'Neill and Mr Namah presented a unified front when the reopening of the Howard-era centre was raised in late 2011, almost nine months before the national election that saw Mr Namah move to the opposition benches.

The PNG court is yet to set a date to hear the challenge, and court officers say the case is unlikely to be heard in January.